The present application is a divisional application of Ser. No. 09/300,389 (xe2x80x9cthe parent applicationxe2x80x9d) filed Apr. 27, 1999 (now allowed) and claims priority to Japanese Application No. JP 10-134335 filed Apr. 28, 1998.
1. Field of the Invention
The present invention relates to an opto-electric conversion device with a new structure, or a light-receiving device.
2. Description of the Related Art
A light-receiving device has been known to have a pin junction structure. A backward voltage is applied to the pin layers of the device, and electron-hole pairs are generated by that light incident from the side of a p-layer is absorbed in an i-layer. The electron-hole pairs excited in the i-layer are accelerated by a backward voltage in the i-layer, and electrons and holes are flowing into an n-layer and a p-layer, respectively. Thus a photocurrent whose intensity varies according to an intensity of the incident light is outputted.
To improve an opto-electric conversion effectivity, the i-layer which absorbs light is formed to have a comparatively larger thickness. But when the thickness of the i-layer becomes thicker, more times are needed to draw carriers to the n-layer and the p-layer. As a result, the response velocity of the opto-electric conversion is lowered. To improve the velocity, an electric field in the i-layer is increased by increasing a backward voltage. But when the backward voltage is enlarged, element separation becomes difficult and a leakage current occurs. As a result, a photocurrent which flows when light is not incident on the device, or a dark current, is increased.
Thus conventional light-receiving devices had an interrelation among a light-receiving sensitivity, a detecting velocity, and a noise current, which restricts their performances.
It is, therefore, an object of the present invention to improve the light-receiving sensitivity and the response velocity Of the opto-electric conversion by providing a light-receiving device having a pin junction of a completely new structure.
In light of these objects a first aspect of the present invention is a light-receiving device, which converts incident light into electric current, constituted by quantum-wave interference layer units having plural periods of a pair of a first layer and a second layer, the second layer having a wider band gap than the first layer, and a carrier accumulation layer disposed between adjacent two of the quantum-wave interference layer units. Each thickness of the first and the second layers is determined by multiplying by an odd number one fourth of a quantum-wave wavelength of carriers in each of the first and the second layers, and the carrier accumulation layer has a band gap narrower than that of said second layer. Plural units of the quantum-wave interference layers are formed with a carrier accumulation layer, which has a band gap narrower than that of the second layer, lying between each of the quantum-wave interference units.
The second aspect of the present invention is to set a kinetic energy of the carriers, which determines the quantum-wave wavelength, at the level near the bottom of a conduction band when the carriers are electrons or at the level near the bottom of a valence band in the second layer when the carriers are holes.
The fourth aspect of the present invention is to define each thickness of the first and the second layers as follows:
DW=nWxcexW/4=nWh/4[2mW(E+V)]1/2xe2x80x83xe2x80x83(1)
and
DB=nBxcexB/4=nBh/4(2mBE)1/2xe2x80x83xe2x80x83(2)
In Eqs. 1 and 2, h, mW, mB, E, V, and nW, nB represent Plank""s constant, the effective mass of carrier conducting in the first layer, the effective mass of carriers in the second layer, the kinetic energy of the carriers at the level near the lowest energy level of the second layer, the potential energy of the second layer relative to the first layer, and odd numbers, respectively.
The fourth aspect of the present invention is a quantum-wave interference layer having partial quantum-wave interference layers Ik with arbitrary periods Tk including a first layer having a thickness of nWkxcexWk/4 and a second layer having a thickness of nBkxcexBk/4 for each of a plural different values Ek, Ek+V. Ek, Ek+V, xcexBk, xcexWk, and nBk, nWk represents a kinetic energy of carriers conducted in the second layer, a kinetic energy of carriers conducted in the first layer, a quantum-wave wavelength corresponding energies of the second layer and the first layer, and odd numbers, respectively.
The fifth aspect of the present invention is to form a carrier accumulation layer having the same bandwidth as that of the first layer.
The sixth aspect of the present invention is to form a carrier accumulation layer having a same thickness as its quantum-wave wavelength xcexW.
The seventh aspect of the present invention is to form a xcex4 layer between the first layer and the second layer, which sharply varies band gap energy at the boundary between the first and second layers and is substantially thinner than that of the first and the second layers.
The eighth aspect of the present invention is a light-receiving device having a pin junction structure, and the quantum-wave interference layer and the carrier accumulation layer are formed in the i-layer.
The ninth aspect of the present invention is to form the quantum-wave interference layer and the carrier accumulation layer in the n-layer or the p-layer.
The tenth aspect of the present invention is a light-receiving device having a pin junction structure.
The principle of the light-receiving device of the present invention is explained hereinafter. FIG. 1 shows an energy diagram of a conduction band and a valence band when an external voltage is applied to the interval between the p-layer and the n-layer in a forward direction. As shown in FIG. 1, the conduction band of the i-layer becomes plane by applying the external voltage. Four quantum-wave interference layer units Q1 to Q4 are formed in the i-layer, and carrier accumulation layers C1 to C3 are formed at each interval of the quantum-wave interference layer units. FIG. 2 shows a conduction band of a quantum-wave interference layer unit Q1 having a multi-layer structure with plural periods of a first layer W and a second layer B as a unit. A band gap of the second layer B is wider than that of the first layer W.
Electrons conduct from left to right as shown by an arrow in FIG. 2. Among the electrons, those that exist at the level near the lowest energy level of a conduction band in the second layer B are most likely to contribute to conduction. The electrons near the bottom of the conduction band of the second layer B have a kinetic energy E.
Accordingly, the electrons in the first layer W have a kinetic energy E+V which is accelerated by potential energy V due to the band gap between the first layer W and the second layer B. In other words, electrons that move from the first layer W to the second layer B are decelerated by potential energy V and return to the original kinetic energy E in the second layer B. As explained above, the kinetic energy of electrons in the conduction band is modulated by potential energy due to the multi-layer structure.
When thicknesses of the first layer W and the second layer B are equal to an order of the quantum-wave wavelength, electrons tend to have characteristics of a wave. The wave length of the electron quantum-wave is calculated by Eqs. 1 and 2 using kinetic energy of the electron. Further, defining the respective wave number vector of first layer W and second layer B as KW and KB, reflectivity R of the wave is calculated by:
R=(|KW|xe2x88x92|KB|)/(|KW|+|KB|)=([mW
(E+V)]1/2xe2x88x92[mBE]1/2)/([mW(E+V)]1/2+[mBE]1/2)=
[1xe2x88x92(mBE/mW(E+V))1/2]/[1+(mBE/mW(E+V))1/2]xe2x80x83xe2x80x83(3).
Further, when mB=mW, the reflectivity R is calculated by:
R=[1xe2x88x92(E/(E+V))1/2]/[1+(E/(E+V))1/2]xe2x80x83xe2x80x83(4).
When E/(E+V)=x, Eq. 6 is transformed into:
R=(1xe2x88x92x1/2)/(1+x)xe2x80x83xe2x80x83(5).
The characteristic of the reflectivity R with respect to the energy ratio x obtained by Eq. 5 is shown in FIG. 3.
And when the second layer B and the first layer W have an s-layers structure, the reflectivity RS of an incident plane of the quantum-wave is calculated by:
RS=[(1xe2x88x92xs)/(1+xs)]2xe2x80x83xe2x80x83(6).
When the condition xxe2x89xa6{fraction (1/10)} is satisfied, Rxe2x89xa70.52. Accordingly, the relation between E and V is satisfied with:
Exe2x89xa6V/9xe2x80x83xe2x80x83(7).
Since the kinetic energy E of the conducting electrons in the second layer B exists near the bottom of the conduction band, the relation of Eq. 7 is satisfied and the reflectivity R at the interface between the second layer B and the first layer W becomes 52% or more. Consequently, the multi-layer structure having two kinds of layers with band gaps different from each other enables reflection of quantum-wave of electrons, which conduct in an i-layer, between the first and second layers.
Further, utilizing the energy ratio x enables the thickness ratio DB/DW of the second layer B to the first layer W to be obtained by:
DB/DW=[mW/(mBx)]1/2xe2x80x83xe2x80x83(8).
When light is incident to the i-layer, electrons excited in conduction bands of the carrier accumulation layers C1, C2 and C3 are accumulated therein. The excited electrons tend to flow to the p-layer by applying the forward voltage, but the electrons do not flow because a reflection condition is satisfied for electrons in the quantum-wave interference layer unit which exists at the side toward the p-layer.
But when the electrons existing in the carrier accumulation layers C1, C2 and C3 are increased, electrons tend to exist in higher level. Then a kinetic energy of the electrons existing in higher level increases, and the electrons are not reflected by the quantum-wave interference layer units because of unsatisfaction of the reflection condition. As a result, the electrons pass the quantum-wave interference layer units Q2, Q3, and Q4 and flow toward the p-layer, and thereby a photocurrent results.
Because a forward voltage is applied to the light-receiving device, driving at a low voltage becomes possible and an element separation become easier. When light is not incident, electrons are reflected in the quantum-wave interference layer units effectively. As a result, an electric current does not occur and a dark current can be substantially lowered. The present inventor thinks that electrons are conducted in the quantum-wave interference layer units as a wave. Accordingly, a response velocity is considered to become larger.
The thicknesses of the first layer W and the second layer B are determined for selectively reflecting the holes or the electrons, because of the difference in potential energy between the valence and the conduction bands, and the difference in effective mass of holes and electrons in the first layer W and the second layer B. In other words, the optimum thickness for reflecting electrons is not the optimum thickness for reflecting holes. Eqs. 3-8 refer to a structure of the quantum-wave interference layer for selectively reflecting electrons. The thickness for selectively reflecting electrons is designed based on the difference in the potential energy of the conduction band and on the effective mass of electrons. Further, the thickness for selectively reflecting holes is designed based on the difference in the potential energy of the valence band and on the effective mass of holes, forming another type of quantum-wave interference layer in an i-layer for reflecting only holes and allowing electrons to pass through.
Accordingly, a quantum-wave interference layer unit which reflects holes and functions as a reflective layer to holes can be formed to connect in series to each quantum-wave interference layer units described above, which reflects only electrons.
The light-receiving device described above having a quantum-wave interference layer unit can have a state not to generate an electric current by reflecting carriers selectively in a range of 0 V to a certain value of a bias voltage. Accordingly, the light-receiving device can be formed by only one of the n-layer and the p-layer in which the quantum-wave interference layer units and the carrier accumulation layer are formed. Alternatively, the light-receiving device can be formed by a pn junction structure, in which the quantum-wave interference layer units and the carrier accumulation layer are formed.
FIG. 4 shows a plurality quantum-wave interference units Ik with arbitrary periods Tk including a first layer having a thickness of DWk and a second layer having a thickness of DBk and arranged in series.
Each thickness of the first and the second layers satisfies the formulas:
DWk=nWkxcexWk/4=nWkh/4[2mWk(Ek+V)]1/2xe2x80x83xe2x80x83(9)
and
DBk=nBkxcexBk/4=nBkh/4(2mBkEk)1/2xe2x80x83xe2x80x83(10)
In Eqs. 9 and 10, Ek, mWk, mBk, and nWk and nBk represent plural kinetic energy levels of carriers conducted into the second layer, effective mass of carriers with kinetic energy Ek+V in the first layer, effective mass of carriers with kinetic energy Ek in the second layer, and arbitrary odd numbers, respectively.
The plurality of the partial quantum-wave interference layers Ik are arranged in series from Il to Ij, where j is a maximum number of k required to form a quantum-wave interference layer as a whole. The carriers existing in a certain consecutive energy range can be reflected by narrowing discrete intervals.
The fifth aspect of the present invention is to form the bandwidth of the carrier accumulation layer to have the same bandwidth as that of the first layer. And the sixth aspect of the present invention is to form the carrier accumulation layer to have a same thickness as its quantum wave wavelength xcexW. As a result, the carriers excited in the i-layer can be confined effectively.
The seventh aspect of the present invention is to form a xcex4 layer at the interface between the first layer W and the second layer B. The xcex4 layer has a thickness substantially thinner than both of the first layer W and the second layer B and sharply varies the energy band profile of the device. The reflectivity R of the interface is determined by Eq. 5. By forming the xcex4 layer, the potential energy V of an energy band becomes larger and the value x of Eq. 5 becomes smaller. Accordingly, the reflectivity R becomes larger.
Variations are shown in FIGS. 5A to 5C. The xcex4 layer may be formed on both ends of every first layer W as shown in FIGS. 5A to 5C. In FIG. 5A, the xcex4 layers are formed so that an energy level higher than that of the second layer B may be formed. In FIG. 5B, the xcex4 layers are formed so that an energy level lower than that of the first layer W may be formed. In FIG. 5C, the xcex4 layers are formed so that a band bottom higher than that of the second layer B and a band bottom lower than that of the first layer W may be formed. As an alternative to each of the variations shown in FIGS. 5A to 5C, the xcex4 layer can be formed on one end of every first layer W.